1. Field of the Invention
The present invention relates to stable hydrolyzed silane aqueous emulsions and their application to various substrates to impart oil and water repellency. More specifically but not by way of limitation, the present invention relates to aqueous emulsions of a fluorocarbon silane and an effective amount of an emulsifier having a hydrophile-lipophile balance, HLB, sufficiently high of retain the fluorocarbon silane in a stable aqueous emulsion in substantially a hydrolyzed state.
2. Description of the Related Art
It is known that hydrolyzable silanes can be formulated as aqueous solutions or emulsions that can be applied to various substrates to impart hydrophobic or water repellency properties. However, for these emulsions to achieve long term storage stability they must be buffered to specific pH ranges to inhibit or prevent the hydrolysis of the silanes in the aqueous medium (see for example, U.S. Pat. No. 4,990,377 and U.S. Pat. No. 4,877,654). It is also known that when hydrolyzable fluorocarbon silanes are applied to various surfaces, they can impart both water repellency and oil repellency to those substrates. However, those hydrolyzable fluorocarbon silanes are applied to the surfaces in the molten state or when dissolved in volatile organic solvents, and must generally be cured by heating with a catalyst to chemically affix the fluorocarbon silane to the substrates (see, U.S. Pat. No. 3,012,006). The use of such volatile solvents are generally harmful to the environment and may be hazardous due to their flammability. Where aqueous solutions or emulsions of the hydrolyzable fluorocarbon silanes are possible, the formulations cannot be stored for long periods of time, especially under a broad range of pH conditions, without undergoing hydrolysis and self-condensation to form insoluble polymeric structures.
It is generally recognized (see, Silane Coupling Agents, E. P Plueddemann, 2 nd. Edition, Plenum Press, NY, 1991; and Silanes And Surfaces, D. E. Leyton, Gordon and Breach Science Publ., NY, 1986) that an important aspect of the durable oil and water repellency that is imparted to surfaces by hydrolyzable fluorocarbon silanes, such as with fluorocarbon alkoxysilanes, is the chemical bonding that occurs between the silane and the active hydrogen functional groups on the substrate. This is achieved by initial hydrolysis of the hydrolyzable groups on the silane to silanol groups, which then undergo condensation with the functionality on the substrate. If there is more than one hydrolyzable group on the silane, multiple silane groups will be formed in the hydrolysis and these silanol groups may condense with the functions on the substrate as well as with adjacent silanol groups attached to the surface. The result is a cross-linked and highly durable fluorocarbon siloxane structure on the surface of the substrate. Accordingly, the highest durability would be expected from the silanes with three hydrolyzable groups on the silicon.